Star

• 1. Introduction

  ›

  A star is a luminous spheroid of plasma held together by self-gravity. The nearest one to Earth is the Sun. Look up on a clear night, and the other points of light hang there as if fixed, deceptively still. Only about 4,000 stars are visible to the naked eye, and every one of them sits inside the Milky Way. Yet the observable universe holds an almost unimaginable number, more stars than all the grains of sand on Earth. So how did people first learn that those tiny lights were furnaces like our own Sun? How does a star catch fire, and what is left behind when the fire goes out? And why does almost every chemical element in your body owe its existence to one of these distant objects? The answers run from ancient Babylonian clay catalogues to iron cores collapsing in a single second.

• 2. Reading The Heavens

  ›

  In 1534 BC, ancient Egyptian astronomers produced the oldest accurately dated star chart known. The earliest star catalogues came from Babylonian astronomers of Mesopotamia in the late 2nd millennium BC, during the Kassite Period. Early observers split the sky into two kinds of objects. Fixed stars held their place on the celestial sphere. Wandering stars, the planets, drifted against that backdrop over days or weeks. The Greeks called these wanderers planetes, meaning wanderer, and tied them to deities whose names survive in Mercury, Venus, Mars, Jupiter, and Saturn.

  Hipparchus, working in the 2nd century BC, compiled a catalogue of 1,020 stars that Ptolemy later drew upon. He is also credited with recording the first nova, a new star. Chinese astronomers, despite the belief that the heavens were immutable, knew that new stars could appear. In 185 AD they became the first to record a supernova, now called SN 185. The brightest stellar event in recorded history, SN 1006, was seen in 1006 and described by the Egyptian astronomer Ali ibn Ridwan along with several Chinese observers. The SN 1054 supernova, watched by Chinese and Islamic astronomers, gave birth to the Crab Nebula.

  Medieval Islamic astronomers gave Arabic names to many stars still in use, and built the first large observatory research institutes to produce Zij catalogues. In 964, the Persian astronomer Abd al-Rahman al-Sufi wrote the Book of Fixed Stars, recording star clusters and the Andromeda Galaxy. In the 11th century the polymath Abu Rayhan Biruni described the Milky Way as a multitude of fragments resembling nebulous stars. The Andalusian astronomer Ibn Bajjah argued the same band was countless stars almost touching, citing a conjunction of Jupiter and Mars he observed in 1106.

• 3. Proving The Stars Are Suns

  ›

  In 1584, Giordano Bruno proposed that the stars were like the Sun and might host their own planets, perhaps even Earth-like worlds. The ancient Greek philosophers Democritus and Epicurus had hinted at the idea earlier, as had the medieval Islamic cosmologist Fakhr al-Din al-Razi. Within a century the notion that stars were other suns had become consensus among astronomers.

  Isaac Newton faced a puzzle: if stars were suns, why did they exert no net gravitational pull on the Solar System? Prompted by the theologian Richard Bentley, he suggested the stars were spread equally in every direction. The Italian astronomer Geminiano Montanari recorded variations in the brightness of the star Algol in 1667, hinting that the heavens changed. Edmond Halley published the first measurements of proper motion in a pair of nearby fixed stars, showing they had shifted since the days of Ptolemy and Hipparchus.

  William Herschel was the first to attempt to map how stars are distributed. During the 1780s he set up gauges in 600 directions and counted the stars along each line of sight. The numbers climbed toward one side of the sky, in the direction of the Milky Way core. His son John Herschel repeated the survey in the southern hemisphere and found the same rise. William Herschel also discovered that some stars are not chance alignments but true physical companions, forming binary systems. The first solution for deriving a binary orbit from telescope observations came from Felix Savary in 1827.

• 4. The Light Decoded

  ›

  Joseph von Fraunhofer and Angelo Secchi pioneered stellar spectroscopy by comparing the spectra of stars like Sirius to the Sun. They found differences in the dark absorption lines, caused by the atmosphere soaking up specific frequencies. In 1865, Secchi began sorting stars into spectral types. The modern classification scheme was developed by Annie J. Cannon in the early 1900s.

  Friedrich Bessel made the first direct measurement of a star's distance in 1838, using parallax to place 61 Cygni at 11.4 light-years. These measurements revealed the vast gulfs between stars. In 1834, Bessel also noticed changes in the proper motion of Sirius and inferred a hidden companion. Edward Pickering found the first spectroscopic binary in 1899, watching the spectral lines of Mizar split on a 104-day period.

  Karl Schwarzschild discovered that a star's color, and therefore its temperature, could be read by comparing its visual magnitude against its photographic magnitude. In 1921, Albert A. Michelson made the first measurement of a stellar diameter using an interferometer on the Hooker telescope at Mount Wilson Observatory. The Hertzsprung-Russell diagram, developed in 1913, gave astrophysics a powerful new tool. Then in her 1925 PhD thesis, Cecilia Payne-Gaposchkin first proposed that stars are made mostly of hydrogen and helium, a claim that reshaped what stars were understood to be.

• 5. Born From The Cloud

  ›

  Stars condense from molecular clouds, regions of space denser than their surroundings yet thinner than a laboratory vacuum chamber. These clouds are mostly hydrogen, with about 23 to 28 percent helium and a few percent heavier elements. The Orion Nebula is one such star-forming region. Most stars form in groups ranging from dozens to hundreds of thousands.

  Gravitational instability begins the process, often triggered when radiation from massive stars compresses a cloud, or when clouds or even galaxies collide. Once a region satisfies the criteria for Jeans instability, it collapses under its own gravity. Dense knots of dust and gas form Bok globules. As a globule contracts, gravitational energy turns into heat and the temperature climbs. When the cloud reaches hydrostatic equilibrium, a protostar forms at the core, often wrapped in a protoplanetary disk.

  The gravitational contraction lasts about 10 million years for a star like the Sun, and up to 100 million years for a red dwarf. The lightest of these young objects are called T Tauri stars, while heavier ones are Herbig Ae/Be stars. They fire jets of gas along their rotation axis, which can shed angular momentum and leave glowing patches called Herbig-Haro objects. Early on, T Tauri stars follow the Hayashi track, dimming while holding roughly the same temperature. A 2017 study of the Perseus molecular cloud found that most newly formed stars sit in binary systems, with the best model suggesting all stars begin as binaries before some pairs split apart.

• 6. The Long Burn

  ›

  Stars spend about 90 percent of their lives fusing hydrogen into helium in their cores, the phase astronomers call the main sequence. The Sun reached this stage 4.6 billion years ago and has brightened by about 40 percent since. As helium builds up in the core, the fusion rate, temperature, and luminosity all slowly climb. Every star also sheds a stellar wind. The Sun will lose about 0.01 percent of its mass over its whole lifespan, while very massive stars can lose over half their mass on the main sequence alone.

  The time a star lasts depends on how much fuel it holds and how fast it burns. The Sun is expected to live 10 billion years. Massive stars devour their fuel and die young, lasting only a few million years. Red dwarfs burn so slowly they can endure for about a trillion years, with the most extreme reaching roughly 12 trillion. Since the universe is only 13.8 billion years old, no red dwarf has yet moved off the main sequence.

  Metallicity, the abundance of elements heavier than helium, shapes a star's fate too. Astronomers call all such elements metals. Metallicity affects how long a star takes to burn its fuel and how its magnetic fields form, which in turn governs the strength of its stellar wind. Older population II stars carry far less metallicity than younger population I stars, because the clouds that made them were poorer in heavy elements. With each generation of dying stars, those clouds grow richer.

• 7. Endings And Aftermath

  ›

  In about 5 billion years, when the Sun begins burning helium, it will swell to roughly 1 AU in radius, 250 times its present size, and lose 30 percent of its mass. Low-mass stars like the Sun become red giants, ignite helium in a sudden helium flash, then drift onto the asymptotic giant branch. There they pulse, ejecting as much as 50 to 70 percent of their mass into space. That material, enriched with carbon and oxygen dredged up from the core, forms a planetary nebula. What remains is a white dwarf, an object about the size of Earth made of electron-degenerate matter.

  Massive stars take a more violent road. A star above 9 solar masses becomes first a blue supergiant, then a red supergiant. Stars beyond 40 solar masses, like Alnilam in Orion's Belt, skip the red supergiant stage and may become Wolf-Rayet stars. As helium runs out, the core fuses carbon, then neon, oxygen, and silicon in onion-layer shells. The final stage arrives when iron forms. Because iron nuclei are more tightly bound than anything heavier, fusing them yields no net energy.

  With its furnace stalled, the iron core grows too heavy to support itself and collapses in a single instant. Electrons are crushed into protons, making neutrons, neutrinos, and gamma rays, and the rebound blows the star apart as a supernova. The blast can briefly outshine an entire galaxy. It leaves behind a neutron star, perhaps a pulsar, or for the largest stars a black hole. The pulsar at the heart of the Crab Nebula spins 30 times every second, slowing gradually as it radiates.

• 8. Star Stuff Returns

  ›

  Stellar nucleosynthesis creates almost every naturally occurring element heavier than lithium. The triple-alpha process turns helium into carbon at cores near 100 million kelvin, using beryllium as a stepping stone. In the Sun's 16-million-kelvin core, hydrogen fuses to helium through the proton-proton chain, while more massive stars run the carbon-nitrogen-oxygen cycle. Each individual reaction yields only a tiny amount of energy, but countless reactions together power the star's glow.

  When stars die, supernova explosions and stellar winds return chemically enriched material to the interstellar medium. Those heavy elements make rocky planets possible, and they are recycled into new stars. The very first stars, formed after the Big Bang with no elements heavier than lithium, may have been enormous. This generation of population III stars likely seeded the universe with the heavier elements needed for planets and life. In June 2015, astronomers reported evidence for such stars in the Cosmos Redshift 7 galaxy.

  Because an AGB star carries energy outward mainly by convection, the matter it throws off is laced with fusion products from deep inside. So future generations of stars are quite literally made of star stuff from past stars. The oldest star yet found, HD 140283, nicknamed the Methuselah star, is estimated at 14.46 billion years old, a figure whose uncertainty keeps it from clashing with the 13.799-billion-year age of the universe measured by the Planck satellite.